Acoustic wave filter device

ABSTRACT

An acoustic wave filter device includes plural filter circuits between an input terminal and an output terminal. Plural inductors are connected in series in a series arm that connects the input terminal and the output terminal, and plural first acoustic wave resonators are connected between the series arm and a ground potential. Each of the filter circuits includes at least one of the inductors and one of the first acoustic wave resonators. A second acoustic wave resonator in the series arm connects adjacent filter circuits. The acoustic wave filter device has a pass band lower than a trap band, steep attenuation characteristics in a range from the pass band to the trap band, and is capable of providing a large amount of attenuation in the trap band.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International ApplicationNo. PCT/JP2007/071887, filed Nov. 12, 2007, which claims priority toJapanese Patent Application No. JP 2006-337116, filed Dec. 14, 2006, theentire contents of each of these applications being incorporated hereinby reference in their entirety.

BACKGROUND

1. Technical Field

An acoustic wave filter device having a pass band lower than a trap bandis disclosed.

2. Description of the Related Art

Up until now, various filter devices using acoustic waves, such assurface acoustic wave devices and boundary acoustic wave devices, havebeen proposed as band filters for communication apparatuses. Forexample, surface acoustic wave devices have been widely used as RF bandfilters for mobile phones.

Recently, digital terrestrial television broadcasting has been becomingwidespread. In digital terrestrial broadcasting, one channel is dividedinto 13 segments. Of the 13 segments, one segment located at the centerof one channel is used as a transmission band for mobile phones.Broadcasting using this one segment is so-called one-segmentbroadcasting, which uses a transmission band from 470 MHz to 770 MHz.

Transmission bands for mobile phones may vary among different methodsand communications companies. For example, a band of 800 MHz or more,such as from 830 MHz to 845 MHz or from 898 MHz to 924 MHz, a 1.7 GHzband, and a 1.9 GHz band are used. Some mobile phones are capable ofreceiving and even recording one-segment broadcasting. In such a mobilephone, if phone transmission is performed simultaneously with receptionand recording of one-segment broadcasting, a received video image may bedistorted due to the effect of transmitted radio waves. Accordingly,there is a demand for band rejection filters having a trap band equal toa transmission band of the mobile phone and having a pass band lowerthan the trap band.

Japanese Unexamined Patent Application Publication No. 2004-129238 (“the'238 application”) discloses an example of a surface acoustic wavefilter device of band rejection type. FIG. 14 illustrates a circuitconfiguration of the surface acoustic wave filter device described inthe '238 application.

As shown in FIG. 14, a surface acoustic wave filter device 501 has aninput terminal 502 and an output terminal 503. An inductor 504 isprovided in a series arm connecting the input terminal 502 and theoutput terminal 503. A surface acoustic wave resonator 505 is connectedbetween one end of the inductor 504 and a ground potential, while asurface acoustic wave resonator 506 is connected between the other endof the inductor 504 and the ground potential. That is, the inductor 504and the two surface acoustic wave resonators 505 and 506 are provided asa π-type filter circuit.

In the surface acoustic wave filter device 501, resonance frequencies ofthe surface acoustic wave resonators 505 and 506 are placed in anattenuation band in intended filter characteristics, an electric signalat the resonance frequencies is lowered to the ground potential, andthus attenuation characteristics are obtained. That is, a trap band isdefined by the resonance frequencies of the surface acoustic waveresonators 505 and 506.

In the surface acoustic wave device 501, the resonance frequencies ofthe surface acoustic wave resonators 505 and 506 are placed in afrequency range where a trap is provided, and thus the trap band isdefined. However, until the electric signal at the resonance frequenciesis lowered to the ground potential, the electric signal has a commoninductance component on a piezoelectric substrate or a package includedin the surface acoustic wave filter device 501. As a result, since thesignal leaks through the surface acoustic wave resonators 505 and 506,satisfactory attenuation characteristics cannot be achieved in the trapband. Therefore, for example, if the surface acoustic wave filter device501 is used as a band rejection filter for a reception stage ofone-segment broadcasting in the mobile phone capable of receivingone-segment broadcasting, it is difficult to reliably attenuatetransmitted radio waves of the mobile phone during reception orrecording of one-segment broadcasting.

SUMMARY

To overcome the problems of the conventional techniques described above,and to provide an acoustic wave filter device having steep attenuationcharacteristics in a region of a trap band near a pass band and capableof providing a large amount of attenuation, embodiments of the inventionprovide an acoustic wave filter device having a trap band and a passband lower than the trap band. The acoustic wave filter device includesa plurality of inductors connected in series in a series arm having andconnecting an input terminal and an output terminal, and a plurality offirst acoustic wave resonators connected between the series arm and aground potential.

In the acoustic wave filter device, plural filter circuits are arrangedin a direction from the input terminal to the output terminal. Each ofthe plural filter circuits includes at least one of the plural inductorsand the first acoustic wave resonators connected between the groundpotential and respective ends of the at least one of the plurality ofinductors.

The acoustic wave filter device further includes a second acoustic waveresonator provided in the series arm. In at least one of areas where theplural filter circuits are adjacent to each other, the adjacent filtercircuits are electrically connected by the second acoustic waveresonator.

In the acoustic wave filter device according to the invention, in everyarea where filter circuits are adjacent to each other, the adjacentfilter circuits are electrically connected by the second acoustic waveresonator.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic plan view of an acoustic wave filter deviceaccording to an exemplary embodiment.

FIG. 2 is a circuit diagram of the acoustic wave filter device accordingto the embodiment shown in FIG. 1.

FIG. 3 is a graph showing impedance characteristics of each acousticwave resonator (AWR) included in the acoustic wave filter deviceaccording to the exemplary embodiment shown in FIG. 2.

FIG. 4 is a schematic circuit diagram illustrating a mechanism forreducing signal leakage in the acoustic wave filter device according toan exemplary embodiment.

FIG. 5 is a circuit diagram illustrating a path along which a signalleaks to an output terminal in an acoustic wave filter device accordingto an exemplary embodiment.

FIG. 6 is a graph showing attenuation frequency characteristics of anacoustic wave filter device according to an exemplary embodiment.

FIG. 7 is a graph showing attenuation frequency characteristics of anacoustic wave filter device according to an exemplary embodiment.

FIG. 8 is a circuit diagram of an acoustic wave filter device accordingto an exemplary embodiment.

FIG. 9 is a graph showing impedance characteristics of plural firstacoustic wave resonators (AWRs) included in the acoustic wave filterdevice of the exemplary embodiment shown in FIG. 8.

FIG. 10 is a graph showing attenuation frequency characteristics ofacoustic wave filter devices according to the exemplary embodiment shownin FIG. 2 and the exemplary embodiment shown in FIG. 8.

FIG. 11 is a graph showing attenuation characteristics corresponding todifferent inductances of inductors in the acoustic wave filter device ofthe exemplary embodiment shown in FIG. 8.

FIG. 12 is a schematic plan view illustrating an exemplary embodiment ofan acoustic wave filter device in which chip coils constitutinginductors are mounted on a piezoelectric substrate.

FIG. 13 is a circuit diagram illustrating an acoustic wave filter deviceaccording to an exemplary embodiment.

FIG. 14 is a circuit diagram illustrating an example of a conventionalsurface acoustic wave filter device.

FIG. 15 is a schematic circuit diagram illustrating a signal leakagepath in the conventional surface acoustic wave filter device.

FIG. 16 is a circuit diagram of an acoustic wave filter device accordingto an exemplary embodiment.

FIG. 17 is a circuit diagram of a multi-stage acoustic wave filterdevice according to exemplary embodiment.

FIG. 18 is a graph showing attenuation frequency characteristics of twoexemplary acoustic wave filter device embodiments.

DETAILED DESCRIPTION

Hereinafter, with reference to the drawings, the invention will be madeapparent by describing exemplary embodiments.

FIG. 1 is a schematic plan view of an exemplary acoustic wave filterdevice 1 according to an embodiment. FIG. 2 is a circuit diagram of theacoustic wave filter device 1 of the FIG. 1 embodiment. The acousticwave filter device 1 is a surface acoustic wave filter device usingsurface acoustic waves.

As illustrated in FIG. 1, the acoustic wave filter device 1 can includea piezoelectric substrate 2. For example, a Y-cut X-propagating lithiumniobate (LiNbO₃) substrate can be used as the piezoelectric substrate 2.Alternatively, the piezoelectric substrate 2 can be a LiNbO₃ substratehaving a different crystal orientation, a different piezoelectric singlecrystal substrate, such as a piezoelectric LiTaO₃ or quartz substrate,or a piezoelectric ceramic substrate.

A circuit configuration illustrated in FIG. 2 can be realized byproviding metal, for example as aluminum (Al) or copper (Cu), or alloyelectrodes on the piezoelectric substrate 2. As illustrated in FIG. 2,the acoustic wave filter device 1 has a series arm connecting an inputterminal 3 and an output terminal 4. A plurality of inductors 5 and 6are provided in the series arm. A plurality of first acoustic waveresonators 7 to 11 are connected such that the series arm is connectedto a ground potential. That is, the first acoustic wave resonator 7 isconnected between the series arm and the ground potential to form oneparallel arm. Similarly, each of the other first acoustic waveresonators 8 to 10 forms one parallel arm.

The embodiment shown in FIG. 2 includes an inductor 5, the firstacoustic wave resonator 7 connected between one end of the inductor 5and the ground potential, and the first acoustic wave resonator 8connected between the other end of the inductor 5 and the groundpotential. The first acoustic wave resonator 7, inductor 5, and firstacoustic wave resonator 8 are provided as a π-type first filter circuit.Similarly, an inductor 6, the first acoustic wave resonator 9 connectedbetween one end of the inductor 6 and the ground potential, and thefirst acoustic wave resonator 10 connected between the other end of theinductor 6 and the ground potential are provided as a π-type secondfilter circuit.

As shown in FIGS. 1 and 2, a second acoustic wave resonator 11 isprovided in the series arm to electrically connect the first and secondfilter circuits. For example, the second acoustic wave resonator 11 canbe provided in the series arm in an area where the first and secondfilter circuits are adjacent to each other, and thus electricallyconnect the adjacent first and second filter circuits to each other.

In FIG. 1, each of the first acoustic wave resonators 7 to 10 and thesecond acoustic wave resonator 11 is schematically represented by arectangular frame. Each of the acoustic wave resonators can be produced,for example, by providing an interdigited (IDT) electrode and a pair ofreflectors (not shown) on the piezoelectric substrate 2. The reflectorscan be arranged on both sides of the IDT electrode in a surface wavepropagation direction, although an electrode structure of each resonator7 to 11 is not particularly limited to this.

As illustrated in FIG. 1, a compact surface acoustic wave filter chipcan be produced by providing the acoustic wave resonators 7 to 11 on onepiezoelectric substrate 2. As shown in FIG. 1, the inductors 5 and 6 areprovided as inductance components external to the piezoelectricsubstrate 2. Alternatively, the inductors 5 and 6 may be provided on thepiezoelectric substrate 2 or provided by providing an inductance elementsection within the piezoelectric substrate 2. Also, one or more of theacoustic wave resonators 7 to 11 may be provided as discrete componentsinstead of integrating two or more of them on a common piezoelectricsubstrate.

FIG. 3 shows impedance characteristics of the first acoustic waveresonators 7 to 10 and impedance characteristics of the second acousticwave resonator 11. In FIG. 3, “acoustic wave resonator” is representedas “AWR” for clarity of illustration. Impedances at resonancefrequencies of the first acoustic wave resonators 7 to 10 (i.e.,parallel arm resonators) and an impedance at an antiresonance frequencyof the second acoustic wave resonator 11 (i.e., series arm resonator)are used to define a trap band, that is, an attenuation band.

As shown in FIGS. 1 and 2, the first and second filter circuits areconnected to each other by the second acoustic wave resonator 11. Aswill be described later in detail, this arrangement can increase theamount of attenuation in a region of the trap band near the pass band,and enhance the steepness of attenuation characteristics.

With reference now to FIGS. 14 and 15, the conventional surface acousticwave filter device 501 cannot achieve satisfactory attenuationcharacteristics. This is probably due to the following: In the surfaceacoustic wave filter device 501 illustrated in FIG. 14, the surfaceacoustic wave resonators 505 and 506 are connected by the inductor 504,in other words, by a reactance component. In this case, the resonancefrequencies of the surface acoustic wave resonators 505 and 506 are usedto obtain attenuation characteristics. However, until the resonancefrequencies of the surface acoustic wave resonators 505 and 506, whichare a plurality of parallel arm resonators, become close to each otherand a signal at the resonance frequencies is lowered to the groundpotential, the signal has a common inductance component between thesurface acoustic wave resonator 505 or 506 and the ground potential. Asindicated by arrows in FIG. 15, this common inductance component causesthe signal at the resonance frequency of the surface acoustic waveresonator 505 to pass through a common inductance and the surfaceacoustic wave resonator 506, and to leak to the output terminal 503.This leads to degradation in attenuation characteristics. An inductanceL in FIG. 15 represents an inductance provided by wiring or the likebetween an electrode land connected to the ground potential of theacoustic wave filter device 1 and the ground potential.

In contrast, the acoustic wave filter device embodiments describedherein have filter circuits that are connected to each other by theacoustic wave resonator. For example, the acoustic wave filter 1 havingthe circuit configuration illustrated in FIG. 2 has the first and secondfilter circuits connected to each other by the second acoustic waveresonator 11, which can be a series arm resonator. In this case, asdescribed above, the attenuation band can be obtained by using theresonance frequencies of the first acoustic wave resonators 7 to 10,which are parallel arm resonators, and the antiresonance frequency of aseries arm second acoustic wave resonator 11.

As shown in FIG. 3, the antiresonance frequency of the second acousticwave resonator 11 is close to the resonance frequencies of the firstacoustic wave resonators 7 to 10. Therefore, although an electric signalat the resonance frequency of the first acoustic wave resonator 7 flowsas indicated by arrows in FIG. 4, this signal path is cut off by thesecond acoustic wave resonator 11. This can reduce signal leakagethrough the first acoustic wave resonators 7 and 8 to the outputterminal. Thus, steep attenuation characteristics with a large amount ofattenuation can be obtained.

An inductance L in FIG. 4 represents an inductance inserted by wiring orthe like between an electrode land connected to the ground potential ofthe acoustic wave filter device 1 and the ground potential.

FIG. 6 and FIG. 7 are graphs showing attenuation frequencycharacteristics of the acoustic wave filter device 1 according to theexemplary embodiment of FIG. 2. As is apparent from FIG. 6 and FIG. 7, alarge amount of attenuation can be obtained at frequencies from 820 MHzto 860 MHz. As shown, steepness of attenuation frequency characteristicscan be achieved in the range from the pass band lower than the trap bandto the trap band.

When the acoustic wave filter device 1 is used in a mobile phone, forexample, as a band filter for a reception stage of one-segmentbroadcasting, a transmission band in an RF stage is often defined as anattenuation band. Examples of the transmission band in the RF stageinclude an 800 MHz band, a 1.7 GHz band, and a 1.9 GHz band. As isapparent from the attenuation frequency characteristics shown in FIG. 6and FIG. 7, a large amount of attenuation can be obtained in any ofthese frequency bands.

Embodiments of acoustic wave filter device may include only first andsecond filter circuits as the plurality of filter circuits. In thiscase, since the first and second filter circuits are electricallyconnected to each other by one second acoustic wave resonator 11, it ispossible to provide a compact acoustic wave filter device having asimple circuit configuration.

In other embodiments, an acoustic wave filter device may include mayinclude three or more filter circuits filter circuits. For example, FIG.13 shows an embodiment in which the acoustic wave filter device 1includes three filter circuits provided between the input terminal 3 andthe output terminal 4.

In the embodiment illustrated in FIG. 13, a third filter circuit isfurther arranged on the input side of the first and second filtercircuits. The third filter circuit is a π-type filter circuit composedof an inductance 12 and first acoustic wave resonators 13 and 14, whichare parallel arm resonators. As shown in FIG. 13, the third filtercircuit is connected to the first filter circuit, with no secondacoustic wave resonator therebetween.

FIG. 16 is a circuit diagram illustrating an embodiment anotherexemplary acoustic wave filter device. In the embodiment shown in FIG.16, as in the case of the embodiment of FIG. 13, a third filter circuitis arranged on the input side of first and second filter circuits of thetype shown in the FIG. 2 embodiment. Again, the third filter circuit isa π-type filter circuit composed of the inductance 12 and the firstacoustic wave resonators 13 and 14, which are parallel arm resonators.In the embodiment of FIG. 16, however, the third filter circuit isconnected to the input side of the first filter circuit via a reactanceelement 15, not an acoustic wave resonator. For example, the reactanceelement 15 may be an inductor or a capacitor.

As is apparent from the modifications illustrated in FIG. 13 and FIG.16, it is only necessary that adjacent filter circuits be connected bythe second acoustic wave resonator 11 in at least one area where filtercircuits are adjacent to each other. Since the second acoustic waveresonator 11 can block the signal having passed through the firstacoustic wave resonators from leaking, it is possible to achieve steepattenuation characteristics and to provide a large amount of attenuationin the attenuation band.

For embodiments including a plurality of filter circuits, a secondacoustic wave resonator can be provided in every area where two filtercircuits are adjacent to each other. FIG. 17 is a circuit diagram ofsuch an embodiment.

FIG. 17 shows a multi-stage acoustic wave filter device provided byconnecting a plurality of acoustic wave filter devices 1 of the FIG. 2embodiment in the direction from the input terminal 3 toward an outputterminal (not shown). FIG. 17 illustrates the acoustic wave filterdevice 1 in the first stage and part of an acoustic wave filter device1A in the next stage. The acoustic wave filter device 1 and the acousticwave filter device 1A are connected to each other via a second acousticwave resonator 11A. That is, the first and second filter circuits of theacoustic wave filter device 1 are connected to each other by the secondacoustic wave resonator 11, and the second filter circuit of theacoustic wave filter device 1 and the first filter circuit of theacoustic wave filter device 1A in the next stage are electricallyconnected to each other by the second acoustic wave resonator 11A. Asdescribed above, in the acoustic wave filter device of the presentembodiment, a second acoustic wave resonator is provided in every areawhere two filter circuits are adjacent to each other. This makes itpossible to more reliably achieve steep attenuation characteristics andprovide a larger amount of attenuation in the attenuation band.

In the above embodiments, two filter circuits are connected to onesecond acoustic wave resonator, resulting in a configuration that makesit possible to produce the acoustic wave filter device 1 withoutsignificantly increasing the number of components. Thus, it is possibleto reduce the number of components, mounting space, and costs.

As described above, a leakage signal path is blocked by the secondacoustic wave resonator 11. As indicated by arrows in FIG. 5 withrespect to the FIG. 2 embodiment, it is possible that a signal havingpassed through the first acoustic wave resonator 7 and the firstacoustic wave resonator 10 on the output terminal side leaks out.However, since the flow of a signal having passed through the firstacoustic wave resonator 7 and the first acoustic wave resonator 8 isblocked as described above, it is possible to achieve steep resonancecharacteristics and realize a large amount of attenuation in therejection band.

As illustrated in FIG. 5, in the acoustic wave filter device 1 of thefirst embodiment, a signal having passed through the first acoustic waveresonator 7 and the first acoustic wave resonator 10 on the outputterminal side may leak out. FIG. 8 is a circuit diagram of an exemplaryacoustic wave filter device 21 that is capable of preventing such signalleakage.

As shown in FIG. 8, the acoustic wave filter device 21 is similar to theacoustic wave filter device 1 of the embodiment shown in FIG. 2 in thatit includes the first acoustic wave resonators 7 to 10, the secondacoustic wave resonator 11, and the inductances 5 and 6.

The acoustic wave filter device 21 is different from the acoustic wavefilter device 1 in that, of the four first acoustic wave resonators 7 to10, the first acoustic wave resonator 10 connected to the outputterminal 3 has a resonance frequency that is different from those of theother first acoustic wave resonators 7 to 9 and is positioned away fromthe trap band of the acoustic wave filter device 21. Thus, in the trapband, the first acoustic wave resonator 10 functions simply as acapacitive element.

FIG. 9 is a graph showing impedance characteristics of the four firstacoustic wave resonators of the embodiment shown in FIG. 2. The legendin FIG. 9 represents “acoustic wave resonator” using “AWR” for clarityof illustration. As shown in FIG. 9, the impedance of the first acousticwave resonator 10 increases at frequencies near the attenuation band.Therefore, it is difficult for an electric signal at the resonancefrequencies of the first acoustic wave resonators 7 to 9 to pass throughthe first acoustic wave resonator 10. Thus, it is possible toeffectively suppress signal leakage along the signal path illustrated inFIG. 5. When compared with the embodiment shown in FIG. 2, theembodiment of FIG. 8 can provide a larger amount of attenuation andachieve steeper attenuation characteristics.

FIG. 10 is a graph showing the attenuation frequency characteristics ofthe acoustic wave filter device 1 according to the embodiment of FIG. 2as a solid line, and the attenuation frequency characteristics of theacoustic wave filter device 21 according embodiment of FIG. 8 as abroken line. As is apparent from FIG. 10, the FIG. 8 embodiment makes itpossible to provide a larger amount of attenuation and achieve steeperattenuation characteristics than those in the case of the FIG. 2embodiment.

As described above, an embodiment can selectively use a plurality offirst acoustic wave resonators, that is, a plurality of parallel armresonators, as a resonator or a capacitive element in the trap band.

According to embodiments, it is possible to provide band filters, suchas band rejection filters, capable of satisfying new market demands forhaving a plurality of trap bands near the pass band. For example, as atransmission band in an RF stage of a mobile phone, a plurality of trapfrequency bands near the pass band of the acoustic wave filter device 21shown in FIG. 8 embodiment may be used.

In this example, the lowest of plural trap frequency bands is set as atrap band of the acoustic wave filter device 21. Then, the other trapbands are defined by positioning, in the remaining trap frequency bands,the resonance frequency of the first acoustic wave resonator used as acapacitive element in the trap band. Thus, plural trap bands can bedefined by varying the resonance frequencies of the plurality ofparallel arm resonators, that is, the plurality of first acoustic waveresonators. It is thus possible to produce a band rejection filterhaving a plurality of trap bands higher than the pass band.

In the embodiment of FIG. 8, attenuation characteristics are less likelyto be changed by variations in an inductance value of the inductors 5and 6. This will be explained with reference to the graph of FIG. 11showing attenuation characteristics observed when the inductance valueof the inductors 5 and 6 varies ±5% in the acoustic wave filter device21. In FIG. 11, a solid line represents filter characteristicscorresponding to a standard inductance value, a broken line representsfilter characteristics observed when the inductance value of theinductors 5 and 6 is 5% higher than the standard inductance value, andan alternate long and short dashed line represents filtercharacteristics observed when the inductance value of the inductors 5and 6 is 5% lower than the standard inductance value. As is apparentfrom FIG. 11, there is almost no effect on filter characteristics evenwhen the inductance value of the inductors 5 and 6, for example,external inductance components, varies ±5%.

Hence, embodiments of an acoustic wave filter device can include atleast one of a plurality of first acoustic wave resonators having aresonance frequency of is outside the trap band. In such a case, sincethe first acoustic wave resonator whose resonance frequency is set to avalue outside the trap band has a resonance frequency outside the trapband, an attenuation band other than the trap band can be provided atthe resonance frequency of the at least one acoustic wave resonator.That is, it is possible to provide a filter device having a plurality ofattenuation bands.

Additionally, the first acoustic wave resonator having the resonancefrequency outside the trap band can be connected between the outputterminal and the ground potential. In this case, the first acoustic waveresonator having the resonance frequency outside the trap band becomescapacitive in the trap band, and an impedance of the first acoustic waveresonator increases. Thus, a leakage signal in the trap band is lesslikely to reach the output terminal via the first acoustic waveresonator having the resonance frequency outside the trap band.Therefore, it is possible to further increase the amount of attenuationin the attenuation band and further enhance the steepness of attenuationfrequency characteristics.

As schematically illustrated in FIG. 12, the inductors 5 and 6 can bechip coil components 5A and 6A mounted on the piezoelectric substrate 2.In such a case, the acoustic wave filter device 1 can be readilyproduced, for example, by surface-mounting the chip coil components onthe piezoelectric substrate. Providing inductance elements on or withinthe piezoelectric substrate 2 makes it possible to omit the process ofmounting external components, such as the above-described chip coils,and can reduce variations in characteristics caused by variations inmounting.

Next, another embodiment of the invention will be described. An acousticwave filter device of this embodiment is similar to the acoustic wavefilter device 1 of the first embodiment in that it includes the firstacoustic wave resonators 7 to 10, the second acoustic wave resonator 11,and the inductances 5 and 6. A circuit diagram of the acoustic wavefilter device according to the third embodiment is identical to thecircuit diagram of the acoustic wave filter device 1 according to theembodiment illustrated in FIG. 2.

The acoustic wave filter device of the present embodiment is differentfrom the acoustic wave filter device 1 in terms of the relationshipbetween the antiresonance frequency of the second acoustic waveresonator 11 (i.e., series arm resonator) and the resonance frequenciesof the first acoustic wave resonators, the antiresonance and resonancefrequencies defining the trap band. That is, the antiresonance frequencyof the second acoustic wave resonator 11 according to the embodimentscorresponding to FIG. 2 is set to a value between the maximum andminimum values among the resonance frequencies of the first acousticwave resonators, the resonance frequencies defining the trap band,whereas the antiresonance frequency of the second acoustic waveresonator 11 according to the present embodiment is set to a value lowerthan the minimum value among the resonance frequencies of the firstacoustic wave resonators. Table 1 shows the resonance and antiresonancefrequencies of the first and second acoustic wave resonators of theacoustic wave filter device according to the present embodiment.

For comparison, Table 2 shows resonance frequency and antiresonancefrequency values of an exemplary acoustic wave filter device that wasproduced according to have a configuration similar to that of theacoustic wave filter device of embodiment of FIG. 2.

TABLE 1 Resonance Frequency Antiresonance Frequency (MHz) (MHz) FirstAcoustic Wave 826 854 Resonator 7 First Acoustic Wave 842 870 Resonator8 First Acoustic Wave 834 859 Resonator 9 First Acoustic Wave 844 872Resonator 10 Second Acoustic Wave 796 821 Resonator 11

TABLE 2 Resonance Frequency Antiresonance Frequency (MHz) (MHz) FirstAcoustic Wave 819 846 Resonator 7 First Acoustic Wave 832 859 Resonator8 First Acoustic Wave 822 846 Resonator 9 First Acoustic Wave 844 872Resonator 10 Second Acoustic Wave 805 832 Resonator 11

FIG. 18 shows a graph 20 of attenuation frequency characteristics of theacoustic wave filter device according the present embodiment in whichthe antiresonance frequency of the second acoustic wave resonator is setto a value lower than the minimum value among the resonance frequenciesof the first acoustic wave resonators. Also shown in FIG. 18 a graph 30of attenuation frequency characteristics of the acoustic wave filterdevice according the embodiment similar to the embodiment of FIG. 2 inwhich the antiresonance frequency of the second acoustic wave resonatoris set to a value between the maximum and minimum values among theresonance frequencies of the first acoustic wave resonators.

As can be seen in FIG. 18, embodiments corresponding to graph 20 exhibitimproved steepness of the attenuation frequency characteristics in therange from the pass band to the trap band when compared with embodimentsthat correspond to graph 30.

In some embodiments, the antiresonance frequency of the second acousticwave resonator is set lower than the minimum value among the resonancefrequencies of the first acoustic wave resonators, the resonancefrequencies defining the trap band, and higher than 0.95 times theminimum value. This is because if the antiresonance frequency of thesecond acoustic wave resonator is too far from the trap band, the amountof attenuation in the trap band becomes small.

Embodiments in which an antiresonance frequency of the second acousticwave resonator is set to be lower than resonance frequencies of thefirst acoustic wave resonators, and where the resonance frequenciesdefine the trap band, result in improved the steepness of attenuationfrequency characteristics in the range from the pass band to the trapband.

Additionally, plural filter circuits can be arranged in a direction fromthe input terminal to the output terminal. Each of the plurality offilter circuits can comprise at least one of the plurality of inductorsand the first acoustic wave resonators connected between the groundpotential and respective ends of the at least one of the plurality ofinductors. Then, in at least one of areas where the plurality of filtercircuits are adjacent to each other, the adjacent filter circuits areelectrically connected by the second acoustic wave resonator. Thus, afractional pass bandwidth as high as 50% or more can be realized, and itis possible to achieve steeper attenuation characteristics near the passband and provide a larger amount of attenuation. For example, on amobile phone, even when transmission takes place during reception orrecording of one-segment broadcasting, it is possible to reliablysuppress distortion of a received video image caused by the transmittedradio waves. That is, it is possible to provide an acoustic wave filterdevice suitable for use as a band rejection filter having a trap bandnear a pass band and required to provide a large amount of attenuationin the trap band.

Although a limited number of embodiments are described herein, one ofordinary skill in the art will readily recognize that there could bevariations to any of these embodiments and those variations would bewithin the scope of the appended claims. For example, the invention isalso applicable to boundary acoustic wave filter devices using boundaryacoustic waves instead of surface acoustic waves. Thus, it will beapparent to those skilled in the art that various changes andmodifications can be made to the acoustic wave filter device describedherein without departing from the scope of the appended claims and theirequivalents.

What is claimed is:
 1. An acoustic wave filter device having a trap bandand a pass band lower than the trap band, the acoustic wave filterdevice comprising: a plurality of inductors connected in series in aseries arm having and connecting an input terminal and an outputterminal; and a plurality of first acoustic wave resonators connectedbetween the series arm and a ground potential, the plurality ofinductors and the plurality of first acoustic wave resonators beingarranged as a plurality of π-type filter circuits in a direction fromthe input terminal to the output terminal, each of the plurality ofπ-type filter circuits being composed of at least one of the pluralityof inductors and the first acoustic wave resonators connected betweenthe ground potential and respective ends of the at least one of theplurality of inductors, the acoustic wave filter device furthercomprising a second acoustic wave resonator having an anti-resonancefrequency provided in the series arm, wherein in at least one ofportions where the plurality of filter circuits are adjacent to eachother, the adjacent π-type filter circuits are electrically connected bythe second acoustic wave resonator, and wherein the anti-resonancefrequency of the second acoustic wave resonator is located in the trapband defined by a resonance frequency of at least one of the pluralityof first acoustic wave resonators.
 2. The acoustic wave filter deviceaccording to claim 1, wherein in every portion where π-type_filtercircuits are adjacent to each other, the adjacent π-type filter circuitsare electrically connected by the second acoustic wave resonator.
 3. Theacoustic wave filter device according to claim 1, wherein the pluralityof π-type filter circuits are first and second filter circuits, and thefirst and second filter circuits are electrically connected to eachother by the second acoustic wave resonator.
 4. The acoustic wave filterdevice according to claim 2, wherein the plurality of π-type filtercircuits are first and second filter circuits, and the first and secondfilter circuits are electrically connected to each other by the secondacoustic wave resonator.
 5. The acoustic wave filter device according toclaim 1, wherein a resonance frequency of at least one of the pluralityof first acoustic wave resonators is outside the trap band.
 6. Theacoustic wave filter device according to claim 2, wherein a resonancefrequency of at least one of the plurality of first acoustic waveresonators is outside the trap band.
 7. The acoustic wave filter deviceaccording to claim 3, wherein a resonance frequency of at least one ofthe plurality of first acoustic wave resonators is outside the trapband.
 8. The acoustic wave filter device according to claim 4, wherein aresonance frequency of at least one of the plurality of first acousticwave resonators is outside the trap band.
 9. The acoustic wave filterdevice according to claim 5, wherein the at least one of the pluralityof first acoustic wave resonators having the resonance frequency outsidethe trap band is connected between the output terminal and the groundpotential.
 10. The acoustic wave filter device according to claim 6,wherein the at least one of the plurality of first acoustic waveresonators having the resonance frequency outside the trap band isconnected between the output terminal and the ground potential.
 11. Theacoustic wave filter device according to claim 7, wherein the at leastone of the plurality of first acoustic wave resonators having theresonance frequency outside the trap band is connected between theoutput terminal and the ground potential.
 12. The acoustic wave filterdevice according to claim 8, wherein the at least one of the pluralityof first acoustic wave resonators having the resonance frequency outsidethe trap band is connected between the output terminal and the groundpotential.
 13. The acoustic wave filter device according to claim 1,further comprising a piezoelectric substrate, wherein the plurality ofthe first acoustic wave resonators and the second acoustic waveresonator are provided on the piezoelectric substrate.
 14. The acousticwave filter device according to claim 1, wherein an antiresonancefrequency of the second acoustic wave resonator is set to be lower thanresonance frequencies of the plurality of the first acoustic waveresonators, the resonance frequencies defining the trap band.
 15. Theacoustic wave filter device according to claim 2, wherein anantiresonance frequency of the second acoustic wave resonator is set tobe lower than resonance frequencies of the plurality of the firstacoustic wave resonators, the resonance frequencies defining the trapband.
 16. The acoustic wave filter device according to claim 3, whereinan antiresonance frequency of the second acoustic wave resonator is setto be lower than resonance frequencies of the plurality of the firstacoustic wave resonators, the resonance frequencies defining the trapband.
 17. The acoustic wave filter device according to claim 4, whereinan antiresonance frequency of the second acoustic wave resonator is setto be lower than resonance frequencies of the plurality of the firstacoustic wave resonators, the resonance frequencies defining the trapband.
 18. The acoustic wave filter device according to claim 5, whereinan antiresonance frequency of the second acoustic wave resonator is setto be lower than resonance frequencies of the plurality of the firstacoustic wave resonators, the resonance frequencies defining the trapband.
 19. The acoustic wave filter device according to claim 9, whereinan antiresonance frequency of the second acoustic wave resonator is setto be lower than resonance frequencies of the plurality of the firstacoustic wave resonators, the resonance frequencies defining the trapband.
 20. The acoustic wave filter device according to claim 13, whereinan antiresonance frequency of the second acoustic wave resonator is setto be lower than resonance frequencies of the plurality of the firstacoustic wave resonators, the resonance frequencies defining the trapband.